If you have a custom enchantment, that say gives you extra health (or any other attribute), there are two weird bugs that I've found can occur:

1. The attribute only gets applied for one piece of armor
2. When taking off armor pieces, the attribute modifiers can be removed completely even while still wearing armor

See attached datapack.

How to recreate:

1. Go into a normal world with the installed datapack.
2. Get any two pieces of trimmable armor
3. Enchant them with their respective "health" enchantments found in the datapack
4. Put on both pieces of armor and observe how only one effect is applied
5. Remove one piece of armor and observe how no effects are applied